问：一张大表有一个字段创建为普通索引(没有重复值)，如果快速把这个字段变为主键索引 答：1.删除索引，创建主键，显然对于大表效率很低。 2.直接在该字段上创建 alter table t add constraint t_id_pk primary key(id); 原理：主键就是生成一个与外键相关的约束，产生一个能保证唯一性的索引。 过程如下： SQL> create table a as select * from dba_objects; Table created. SQL> create index idx_a on a(object_id); SQL> alter table a add constraint a_id_pk primary key(object_id); Table altered. 如果该字段索引不唯一，会怎么样 SQL> create table b as select * from dba_objects; Table created. SQL> insert into b select * from dba_objects; 89346 rows created. SQL> commit; Commit complete. SQL> create index idx_b on b(object_id); Index created. SQL> alter table b add constraint b_id_pk primary key(object_id); alter table b add constraint b_id_pk primary key(object_id) * ERROR at line 1: ORA-02437: cannot validate (T1.B_ID_PK) - primary key violated